In order to reduce noises and power consumption in an apparatus for controlling a fan motor used in an engine cooling system in an automobile, for example, a pulse width modulation (PWM) control module is used to execute continuous or proportional control on the rotating speed of the fan motor. When an excessively large current flows due to a rotation lock state of the fan motor or a short circuit, it is quite likely that a semiconductor switch device in the PWM control module is damaged by heat.
As a general solution, an over-current protection circuit capable of coping with such an excessively large current is thus provided. When the semiconductor switch device is put in a state of being turned off continuously as it is after detection of the flow of the excessively large current, however, the fan motor is also put in a stopped rotation state as it is, having an adverse effect on the function to cool the engine. In order to solve these problems, a motor control apparatus having over-current protection functions has been provided.
For instance, a circuit is provided to establish a limited current state of limiting the current flowing through the fan motor to a level, which does not generate excessive heat causing the breakdown of the semiconductor switch device, when an excessively large current flows due to a rotation lock state of the fan motor or another cause while maintaining continuous conduction of the current through the fan motor in the limited current state as is disclosed in documents such as Japanese Patent No. 3,102,355 (patent document 1).
In addition, another circuit is also adopted to intermittently continue the output of the PWM control module when an excessively large current flows. This circuit is adopted to provide a control configuration in which the current flowing through the fan motor is put in a suppressed state when an excessively large current is detected. In this current suppression state, the temperature of the inside of the PWM control module is monitored. When the temperature of the inside of the PWM control module exceeds an upper limit, the output of the PWM control module is once discontinued. As the temperature of the inside of the PWM control module later becomes lower than a level set in advance, the PWM control module is reset to an initial condition. These operations are carried out repeatedly until a normal condition is restored as disclosed in Japanese patent laid open publication No. Hei 9-284,999 (patent document 2).
The control according to the patent document 1 is effective in maintaining the operation of the fan motor and the prevention of the breakdown of the semiconductor switch device. Since the current conduction through the fan motor is continued, however, it is quite likely that a layer short circuit is generated in the fan motor eventually in dependence on the location of the cause of the excessively large current.
In the case of the control according to the patent document 2, on the other hand, an over-current protection operation is carried out by monitoring the current flowing through the fan motor and the temperature of the inside of the PWM control module at the same time. Thus, in particular, a time delay unavoidably occurs in an operation to stop the output of the PWM control module when the temperature of the inside of the PWM control module exceeds the upper limit. When such a time delay occurs, the temperature of the inside of the PWM control module most likely increases sharply due to a layer short circuit generated in the fan motor or another cause. An inability to cope with such a sharp increase in temperature will lower reliability of the operation to protect the motor control apparatus and the motor from damage.